Sailor Moon is known as one of the most famous examples of the "Magical Girl" genre of anime. Sporting a cast of figuratively and literally colorful characters making up the Sailor Scouts, there's just about a character for anyone out there. One of the most popular among them is Ami Mizuno, also known as Sailor Mercury.
With her power over water and high IQ, Sailor Mercury has been an essential member of the Sailor Scout team and has gotten them through some rough spots. So much so, that there are quite a few characters from other anime that she could defeat with no problem. At the same time though, it's clear she's not invincible and there are just as many characters who could just as easily beat her too.
10 LOSE TO: EUCLIWOOD HELLSCYTHE (IS THIS A ZOMBIE?)
Eucliwood Hellscythe is a silent but powerful Necromancer from the comedy series Is This a Zombie?, but her powers are no laughing matter. The Eucliwood is an immortal being who has the power to revive the dead, summon monsters known as "Megalos" and, most frighteningly, and warp reality to make even the smallest whisper from her lips come true.
Eucliwood's weakness seems to only be the pain from using her own power, but the limits of Eucliwood's magic are unknown. If allowed to run rampant, she could effectively be invincible against any enemy who dares to fight her. While Sailor Mercury is adept with magic in her own right, she might not be able to beat someone who can kill anyone with a single word.
9 CAN DEFEAT: ARMIN ALERT (ATTACK ON TITAN)
The one thing Armin of Attack on Titan and Ami have in common is their high intelligence. Each character acts as the brains of their respective groups and they helped form strategies that made the difference between victory and defeat. Unfortunately for Armin, Sailor Mercury might just be able to outpace this member of the Survey Corps.
Armin would only have a chance as the Colossal Titan, but there's a chance this can't save him either. The Colossal Titan wouldn't be able to dish out the damage to kill Sailor Mercury and she was able to survive attacks from the powerful Eternal Sailor Scouts. Along with her speed, it would only take a matter of time before Sailor Mercury could deduce Armin's weakness at the back of his neck and strike there.
8 LOSE TO: KEFLA (DRAGON BALL SUPER)
Saiyans are powerful warriors and their pension for breaking limits has defied expectations in many cases. While Sailor Mercury could likely defeat the hot-headed Super Saiyan, Caulifla, her chances take a dive when she fuses with Kale to become Kefla.
Kefla was strong enough to keep up with and overwhelm Goku even in Ultra Instinct, who in the forms prior to that was able to contend with gods. Considering that Kale is her universe's Legendary Super Saiyan, this also means that Kefla could possibly have the space-time manipulation powers that Broly had. With that in mind, Kefla could most likely defeat Sailor Mercury through pure brute strength and raw power.
7 CAN DEFEAT: SABER (FATE SERIES)
Artoria Pendragon, also known as Saber, is one of the Fate Series' most skilled and powerful servants. Armed with the legendary Excalibur, Saber can defeat almost anything that comes her way. However, she might not be able to come out on top when up against Sailor Mercury.
While Saber is resistant to many forms of magic, which would give her a good defense against a lot of Mercury's attacks, her offense isn't quite potent enough to defeat the Sailor Scout. Saber's Noble Phantasm's don't exceed the destructive force of the attacks that Sailor Mercury has endured, given that it's likely that she could outlast Saber until her supply of mana runs out, theoretically defeating her without having to land a single hit.
6 LOSE TO: BOBOBO (BOBOBO-BO BO-BOBO)
While most of the contenders on this list have a certain set of rules within their own universe, limiting what they can and can't do, Bobobo-bo Bo-bobo doesn't seem to follow rules or logic of his universe and likely nobody else's either.
Bobobo usually defeats his enemies through not only his brute strength and powerful nose hair attacks (seriously), but also through pure nonsense shenanigans. This allows Bobobo to attack and withstand attacks at ridiculously high levels that are practically impossible to even achieve. Sailor Mercury wouldn't even be able to outwit this afro-ed goof since anything related to sense goes out the window while fighting him. It's hard to contend with someone who can break reality whenever he feels like it.
5 CAN DEFEAT: SAKURA HARUNO (NARUTO & BORUTO)
While Sakura Haruno did start out as one of the weakest ninjas in the world of Naruto, she eventually climbed her way up until she was one of the greatest medical ninjas in the world and it didn't hurt that she had superstrength either. Unfortunately, she would still be outclassed by some enemies and this would be the same if up against Sailor Mercury.
While Sakura is strong enough to lift trees with ease, Sailor Mercury has faced opponents just as powerful and more powerful than Sakura and won.
4 LOSE TO: SAITAMA (ONE-PUNCH MAN)
And some think Superman is overpowered. Saitama, a self-proclaimed "hero for fun", is the very definition of undefeated, and with a single punch that can destroy literally anyone, that is not in question.
Saitama's striking power is an absurdly high level, to the point where even one of his punches changed the weather in a matter of seconds and durable enough to survive the vacuum of space without protective gear. Keep in mind he obtained all this power through a normal workout regimen and most of his feats were when he is barely trying.
3 CAN DEFEAT: CURE HAPPY (SMILE PRECURE/GLITTER FORCE)
Miyuki Hoshizora, also known as Cure Happy, is the cheerful and optimistic protagonist of the magical girl anime, Smile PreCure (or Glitter Force in the dub), and has had some impressive feats of power. With a variety of magic at her fingertips, she was able to resist time-freezing abilities, survived a hit from the "Bad End Cannon" and helped defeat the evil Emperor Pierrot.
Despite all her accomplishments though, she pales in comparison to Sailor Mercury's. One of her most notable feats of power was when she was able to dispel a nightmare dimension (which are implied to be a universe unto themselves). Sadly, the fact is Cure Happy would not have been able to defeat Emperor Pierrot without the help of her team (a villain who also falls short of what the Sailor Scouts are capable of), meaning she would have little chance of overpowering Mercury.
2 LOSE TO: MADOKA KANAME (MADOKA MAGICA)
There might not be a better match up for one of the Sailor Scouts than the magical girl Madoka Kaname. In her basic magical girl form, Madoka has many different abilities that can match blows with anything the Sailor Scouts have in their arsenal, but the notion of a fair fight slowly disappears when comparing any of them to Madoka's ultimate form.
Ascending to heights that would make even gods blush, Madoka is an omnipresent being who can manipulate time, space, and the realities of multiple universes on a whim. As a being who both exists as a concept and doesn't at the same time, even hurting Madoka in this state is impossible without similar abilities. Sailor Mercury would be nothing by comparison.
1 CAN DEFEAT: SAKURA KINOMOTO (CARDCAPTOR SAKURA)
Another wild famous magical girl, Sakura Kinomoto is skilled despite her young age and quite possibly the strongest spellcaster ever, even exceeding Clow Reed. On top of her many cards, all with a variety of powers, Sakura can see the future, has keen senses for magic, and can even create new cards known as "Clear Cards".
Sakura's magical power is immensely strong, but not quite on the level that Sailor Mercury or any of the Sailor Scouts are. Her feats are tremendous in the context of what goes on in Cardcaptor Sakura, but most effective in that context.